Valve actuation in an internal combustion engine can take many forms. Generally, during combustion, intake valves may be opened to admit air into a cylinder to combine with fuel for combustion. One or more exhaust valves may be opened to allow exhaust gases, resulting from combustion, to escape from the cylinder. Intake, exhaust or auxiliary valves may be opened at various times to recirculate gases for improved fuel economy and emissions.
It is known in the art to utilize a chain or belt between a sprocket or pulley on an engine crankshaft and a sprocket or pulley on a camshaft sprocket to turn at least one associated camshaft. A camshaft generally has several lobes along its length, the lobes designed such that each revolution of the camshaft causes a lobe to come in contact a valve actuation element. This element can take many forms known in the art, including a rocker arm or roller finger follower, for example. This valve actuation element converts the rotational motion of the camshaft into linear movement of the valves.
it is further known in the art to utilize more than one valve per intake or exhaust of each cylinder. Such multi-valve operation improves air flow of intake and exhaust gases, potentially improving combustion efficiency, power and performance. Where two or more valves are used in an intake or exhaust valve arrangement, it is known to utilize a valve bridge to activate two valves by one lever or valve actuation element. The force from the cam lobe is transferred through the valve actuation element and onto a contacting pad located equidistantly from the motion plane of each valve in contact with the particular valve bridge. This enables the theoretical value of the moment arm to the two contact areas of the two or more valve stem tips to the valve bridge contact pad to be equal, therefore generating an identical valve motion and valve stem tip contact force. This contacting pad may he integral to the valve bridge or may be formed as a separate button.
However, in actual operation, the valve bridge may apply force to the valve stems unevenly or the valves may be actuated out of unison. This imbalanced movement of the valve bridge can be caused by many factors, for example, the geometric and assembling tolerances of the various components of the system may cause the contact positions of the valve stem and valve bridge to deviate from their intended positions. In addition, the valve spring preload and spring rates may vary by 10% or more, introducing uneven movement of the two valves.
This uneven movement or actuation of the valves often is undesirable because it can contribute to reduced life from increased and more varied valve seating velocity. In extreme cases, the valve stem may break due to edge loading at the valve stem tip owing to substantially uneven loading.